My Galaxy S3 is aging now. That is running Android 4.1 which is quite old. I flashed my phone with a new ROM. I chose OmniRom. That is very lightweighted and simple.
1.I turned off the phone, and then Press Power+Home+ Volume Down to enter a download mode.
2. I used Odin3 to flash the CF-Auto Root image, making the phone to be rooted. You can find those instructions in xda forum.
3. I used ClockwiseMod to flash OmiRom rom.
That is easy.
I got a "new" phone with Android 4.4!

I found the UI is basic and clean. Also, that is very smooth! The most important is the batter life improved a lot!